Composite flywheel rim having commingled layers with macroscopically uniform patterns of fiber arrangement and methods for manufacturing same

ABSTRACT

A composite-based rim includes multiple fiber-based, commingled layers wherein the strength and/or stiffness increases from the innermost to the outermost layer of the rim, but wherein the radial stress and strain generated in the rim decreases from the innermost to the outermost layer. At least some layers have a mixture of carbon fiber tows and glass fiber tows. The ratio of carbon fiber tows to glass fiber tows in each layer is constant, and the distribution of carbon fiber tows is macroscopically uniform in each layer.

CROSS REFERENCE TO RELATED APPLICATIONS

This application claims priority to, incorporates by reference theentirety of, and is being filed as a continuation-in-part of U.S. patentapplication Ser. No. 09/952,151, which was filed on Sep. 13, 2001, andwhich is currently pending, and this application also claims priorityto, incorporates by reference the entirety of, and is being filed as acontinuation-in-part of U.S. patent application Ser. No. 09/952,283,which was filed on Sep. 13, 2001, and which also is currently pending.

FIELD OF THE INVENTION

The present invention relates generally to composite-based rims, andmethods for their manufacture. More particularly, the present inventionrelates to composite-based rims that are ideally suited forincorporation into flywheel systems because the rims are comprised of aplurality of commingled fiber layers, and more specifically to hybridcomposite-based rims that are comprised of a plurality of commingledfiber layers having macroscopically uniform fiber distribution.

BACKGROUND OF THE INVENTION

Flywheel systems have been known in the art for a number of years, andhave proven to be extremely useful in industrial settings (as, forexample, uninterruptible power supplies) due to their excellent abilityto store and recover kinetic energy. A typical flywheel system includesa flywheel, a shaft to which the flywheel is secured, as well as one ormore bearing assemblies that rotatably support the shaft. A flywheelsystem also includes a protective outer rim, which is supported by a hubthat serves to connect the rim to the shaft.

In operation, a high-powered, high-strength motor drives the shaft,which itself drives a rotor at a high velocity. This causes the rim ofthe flywheel system to rotate/spin rapidly, which, in turn, creates asignificant amount of kinetic energy in accordance with the followingequation:Energy=½*(rim density)*(rim volume)*(rotor radius ofgyration)²*(rotational speed of rim)²

Since the advent of flywheels, those in this art have constantly aimedto design a flywheel system that is able to generate as much kineticenergy as possible according to this equation without compromising thesafe operation of the flywheel system. To that end, several years ago,designers began to experiment with switching from metal-based tocomposite-based rims.

Metal-based rims had proven problematic in use because their somewhatlow yield strength limited their ability to generate rotational speedand, therefore, the ability of the flywheel system to generatesignificant amounts of kinetic energy. And although metal-based rims hadproven highly failure resistant, when they did fail, they tended tobreak into three large, heavy pieces, which were jettisoned from theflywheel system, thus presenting a danger to surrounding persons andproperty alike.

Composite-based rims are not only lighter than metal-based rims, but canhave comparable or even higher strengths and stiffnesses, thus allowingthem to achieve much higher rotational speeds and, therefore, toapparently provide most, if not all of the benefits of metal-based rims,without the aforementioned risks/drawbacks.

Not surprisingly, within just a few years of their discovery, compositebased rims had become a widely accepted standard within the flywheel andenergy storage system industry.

More recently, however; it has become evident that composite-based rimsalso can encounter problems in use, chief among which is theirsusceptibility to failure due to radial stresses and strains that ariseduring operation of a flywheel system.

As noted above, flywheel systems that incorporate composite-based rimsare primarily advantageous as compared to metal-based rims because theirlower weight allows them to be able to rotate more rapidly thanmetal-based rims and, in turn, to generate more energy for storage thanwould be generated by an otherwise identically dimensioned metal-basedrim. But as the speed of rotation of any flywheel system rim (whethermetal- or composite-based) increases, so too does the undesirablestrain, and hoop/radial stresses placed against it according to theequations:Hoop Stress=(rim density)*(rim radius)²*(rim rotational speed)²=(hoopstrain)*(hoop modulus)Radial Stress˜(rim density)*(rim Thickness)²*(rim rotational speed)²

Thus, for example, according to the second equation, a first rotatingrim that is twice as thick as a second rotating rim will have a peakradial stress that is approximately four times greater than that of thesecond rotating rim. This is a marked increase in the generation ofstresses and strains in a composite-based rim, which has a relativelylow radial strength as compared to its hoop strength.

Realizing this, but not wanting to sacrifice the benefit(s) of increasedrim rotational speed (and, thus, increased kinetic energy), somesuggested reducing the thickness of composite-based rims, whileincreasing the rims' length.

The likely rationale for doing so was the fact that the square of therim thickness is proportional to the amount of strain and radial stressencountered in the rim, such that a decreased rim thickness shouldoffset enough of the increase in rotational speed of the rim to keep theamount of strain and radial stress encountered in the rim within amanageable range. The length of the rim (which, is a factor of the rimvolume, is proportional to the amount of kinetic energy produced by therim) was increased was to compensate for the reduction in energy thatwould be caused by reducing the rim's cross sectional area, which isalso directly proportional to the amount of kinetic energy generated bythe rim.

Unfortunately, flywheel systems that incorporated rims with both reducedthickness and increased lengths proved to be unduly expensive to produceand to implement and operate, and, therefore, quickly grew out of favorin the art.

Therefore, a need remains for a composite-based rim for use in flywheelsystems, wherein the design of the rim positively influences the abilityof the rim to generate energy without negatively influencing, due to thegeneration of unmanageable radial stresses and strains, the rim'slongevity and the safe operation of a flywheel system within which therim is incorporated.

SUMMARY OF THE INVENTION

The present invention features composite-based flywheel system rimscomprised of a plurality of commingled fiber layers, as well as methodsfor manufacturing such rims that meet this and other needs. Although thecomposite-based rim of the present invention is primarily described asbeing applicable to flywheel-based evacuated energy storage systems, itmay be used in other environments in which stresses and strains areencountered, and are sought to be manageably controlled.

In an exemplary aspect of the present invention, a composite-based rimis comprised of a plurality of tailored, commingled fiber-based layers.The rim should include at least two fiber layers, and may include aplurality of layers up to, or even greater than ten. In a particularlyexemplary aspect of the present invention, the rim includes five layers,at least two of which are commingled layers.

As used herein the terms “co-mingled” and “commingled” as used hereinshall be understood to mean or describe a fiber arrangement thatexhibits a pattern in which there is mingling of non-uniform fibers(e.g., mingling of “low” strength/stiffness fibers such as E-glassand/or “high” strength/stiffness fibers such as carbon) such that eachrim layer comprised of the mingled fibers possess uniform properties.

Each rim layer is generally comprised of a different combination offiber(s) than the other layer(s) such that the rim exhibits increased anstrength to density ratio and/or stiffness in each of its successivelayers from its innermost layer to its outermost layer.

The rim layers include “low” strength/stiffness fibers and/or “high”strength/stiffness fibers, wherein the volume percentage of “low”strength and/or stiffness fiber(s) contained in each layer successivelydecreases or remains constant from the innermost rim layer to theoutermost rim layer, while the volume percentage of “high” strengthand/or stiffness fiber(s) contained in each layer successively increasesor remains constant from the innermost layer to the outermost layer.

In a particular aspect of the invention, the rim includes five layers,wherein the volume percentage of “low” strength/stiffness fiber(s)decreases from the first layer of the rim, to the second layer, to thethird layer, to the fourth layer, and remains constant in the fourth andfifth layers, but where the volume percentage of “high”strength/stiffness fiber(s) increases from the first layer, to thesecond layer, to the third layer, to the fourth layer, and remainsconstant in the fourth and fifth layers.

The specific compositions of the layers of the rim are selected totailor the strengths and stiffnesses of the rim, and, more particularly,to create a smooth gradient of radial stress and strain from layer tolayer.

By varying the composition of the layers, not only are the strength andstiffnesses of those layers varied, so too are their densities andmoduli, both of which affect the generation of radial stress an strainon the rim, which, in turn, affect the formation of undesirable crackswithin the rim.

Further, by smoothly varying the stiffnesses and densities of thelayers, the composite-based rim became, in effect, a radial successionof thin rings spinning together, with the inner rings desirably loadingthe outer rings in slight radial compression. Conventionalcomposite-based rims do not have compositions that allow them to achievethis type of smooth radial stress and strain gradient, and thus are lessresistant to crack formation, and less effective to guard against rimfailure than composite-based rims of the present invention.

Moreover, even in the unlikely event that a crack does form in a rim ofthe present invention, the crack will likely form in the innermost layerof the rim, and will be inhibited from propagating into and through theother four layers of the rim. And even if the crack does manage topropagate, that will cause a flywheel system upset/imbalance condition,which will be sensed and acted upon before the crack has time topropagate through all of the layers of the rim.

Therefore, the above-described technique relating to the composition ofa rim in accordance with the present invention represents a designphilosophy that deters crack formation in the rim by controlling thestresses encountered within the rim to manageable levels. Further, sucha technique, by virtue of the tailored compositions of the layers,causes cracks (if any are formed) to be initially formed in the firstlayer, and severely inhibits the ability of such cracks to propagateinto the second, third, and especially fourth and fifth layers of therim. This, in turn, allows a flywheel system that incorporates acomposite-based rim in accordance with the present invention to beconfidently operated at high speeds without fear of adverse effects(i.e., rim failure/burst), thus allowing for the flywheel system tobeneficially generate a comparatively large amount of kinetic energy.

In additional aspects of the present invention there is featured acomposite flywheel rim having multiple rim layer, more particularlymultiple hybrid fiber layers in each of which the mixture ratio of“high” strength/stiffness fibers such as carbon fibers versus “low”strength/stiffness fibers such as E-glass or glass fibers is constantand the ratio incrementally increases layer by layer toward outside ofthe rim and the distribution of the “high” strength/stiffness fibers orcarbon fibers is macroscopically uniform in each layer.

It has been found that more macroscopically uniform fiber distributionmay be important to achieve uniform stress distribution during rotorspinning even with the constant mixture ratio between “low”strength/stiffness fibers such as E-glass or glass fibers and/or “high”strength/stiffness fibers such as carbon fibers. It shall be understoodhereinafter that reference to carbon fibers shall be understood toinclude “high” strength/stiffness fibers and the reference to e-glassfiber or glass fiber shall be understood to include “low”strength/stiffness fibers.

The macroscopically uniform distribution can be achieved by controllingthe correlation between lead rate of fiber band per mandrel revolutionand the winding length. Carbon fiber tow spacing and position in theband, and a width of a carbon fiber tow also affect the lay up pattern,however, the most effective and the easiest way to change the lay uppattern with constant parameters is by controlling the winding length.

The present invention also features other methods for manufacturing acomposite-based rim, such as via a filament winding technique. Inaccordance with an exemplary aspect of this method, fiber tows arestored within, and dispensed from one or more racks/holders within adevice (e.g., a creel) and are layered atop each other in predeterminedcommingled tow arrangements in order to preliminarily form each of thefiber layers of the rim.

This preliminarily formed layer is directed through one or more physicaladjustment devices (e.g., one ore more rollers and/or one or more combs)and then into a resin treatment area of the apparatus where a spinningdrum is continuously being impregnated with wet resin being held withina holding area (e.g., a resin bath). As the layer advances through theresin treatment area, the resin-coated drum spins in the direction ofadvancement of the layer, thus causing the bottom-facing side of thefiber layer to be coated (by the drum) with wet resin mixture.

The wet layer is then directed through one or more additional physicaladjustment devices (e.g., one or more rollers and/or one or more combs)to produce a uniform, wet layer of fiber material or predeterminedbandwidth. Thereafter, the layer is fed through a guiding device (e.g.,an eyelet) and caused to be wound onto a shape-instilling shell to formone of the layers of the multiple-layer rim of the flywheel system. Onceall the desired layers of the rim have been wound onto the shell, theshell is cured, thus causing the resin on each layer to dry.

In additional aspects, there is featured methods for manufacturing acomposite rim as described herein having multiple hybrid fiber rimlayers. Such a method includes having the fiber tows being laid in alay-up pattern that is defined by controlling the correlation betweenlead rate per mandrel revolution and winding length.

Other aspects and embodiments of the present invention are discussed indetail below.

BRIEF DESCRIPTION OF THE DRAWINGS

For a fuller understanding of the nature and desired objects of thepresent invention, reference is made to the following detaileddescription, which is to be taken in conjunction with the accompanyingdrawing figures wherein like reference characters denote correspondingparts throughout the several views presented within the drawing figures,and wherein:

FIG. 1A is a schematic view of a composite-based rim in accordance withthe present invention;

FIG. 1B is a cross sectional view of the rim of FIG. 1A along the lineA-A;

FIG. 2 is a schematic view of an apparatus for manufacturing the FIG. 1rim;

FIG. 3 is a schematic plan view of a mandrel on which is being wound aband of resin-impregnated fiber tows, illustrating the position of thefiber band at its two extreme end positions;

FIG. 4 is a sectional elevation of the mandrel shown in FIG. 3 after thefiber winding operation has been completed;

FIG. 5 is a sectional diagram of a magnified cross section of compositeflywheel rim, cut along a radial plane parallel to the axis of the rim,illustrating an undesirable stacked fiber distribution;

FIG. 6 is a sectional diagram of a magnified cross section of compositeflywheel rim, cut along a radial plane parallel to the axis of the rim,illustrating a macroscopically cross hatched fiber distribution; and

FIG. 7 is a sectional diagram of a magnified cross section of compositeflywheel rim, cut along a radial plane parallel to the axis of the rim,illustrating a preferred macroscopically random or uniform distributionof carbon fibers amongst the glass fibers.

DETAILED DESCRIPTION OF THE INVENTION

FIGS. 1A and 1B depict an exemplary composite-based rim 100 inaccordance with the present invention. Incorporation of this rim 100into a high stress and strain usage environment, such as a flywheelsystem, allows the flywheel system to be spun at high speeds in order togenerate high levels of kinetic energy while managing the amounts/levelsof strain and radial stresses generated within the rim, and, in turn,minimizing or at least reliably controlling the formation andpropagation of cracks within the rim.

The rim 100 is comprised of a plurality of tailored, commingledfiber-based layers, each of which is spun or wound atop another layervia techniques such as the exemplary filament winding techniquedescribed below and depicted in FIG. 2. The rim 100 includes at leasttwo fiber layers, and may include a plurality of layers up to, or evengreater than ten. Each layer is generally comprised of a differentcombination of fiber(s) than the other layer(s) such that the rimexhibits increased strength and/or stiffness in each of its successivelayers from its innermost layer to its outermost layer. Also, eachsuccessive layer of the rim from its innermost layer to its outermostlayer has a lower density.

The exemplary rim 100 of FIGS. 1A and 1B is comprised of five layers—afirst, innermost layer 110, a second layer 120, a third layer 130, afourth layer 140, and a fifth, outermost layer 150. A gap 160 is definedwithin the inner layer 110 of the rim 100, and will ultimately house,for example, the shaft (not shown) and hub (not shown) of a flywheelsystem.

Because the rim 100 exhibits increased strength and/or stiffness in eachof its successive layers, the fifth, outermost layer 150 of the rim 100is stronger and/or stiffer than the rim's fourth layer 140, which isstronger and/or stiffer than the third layer 130 of the rim, which isstronger and/or stiffer than the rim's second layer 120, which isstronger and/or stiffer than the first, innermost layer 110 of the rim.

To that end, and in accordance with an exemplary embodiment of thepresent invention, the first, innermost layer 110 of the rim 100 iscomprised almost entirely of a comparatively “low” strength and/orstiffness fiber. The fifth, outermost layer 150 of the rim 100 and thefourth layer 140 of the rim, however, are both comprised entirely of afiber with comparatively “high” strength and stiffness characteristics,wherein the strength and/or the stiffness of the fiber that comprisesthe fifth layer is “higher” than the “high” strength and/or stiffnessfiber that comprises the fourth layer.

The volume percentage of “low” strength/stiffness fiber(s) contained ineach layer 110, 120, 130, 140, 150 of the rim 100 successively decreasesor remains constant from the rim's first layer 110 to its fifth layer150, while the volume percentage of “high” strength/stiffness fiber(s)contained in each layer increases or remains constant from the firstlayer to the fifth layer.

Preferably, the volume percentage of “low” strength/stiffness fiber(s)decreases from the first layer 110, to the second layer 120, to thethird layer 130, to the fourth layer 140, and remains constant in thefourth and fifth layers 140, 150, while the volume percentage of “high”strength/stiffness fiber(s) increases from the first layer, to thesecond layer, to the third layer, to the fourth layer, and remainsconstant in the fourth and fifth layers.

More specifically, each of the second and third layers 120, 130generally contains at least 20% by volume of both “low” and “high”strength and/or stiffness fiber materials, wherein the second layer 120includes a greater volume percentage of “low” strength and/or stiffnessfiber(s) than “high” strength and/or stiffness fiber(s), and wherein thethird layer includes a greater volume percentage of “high” strengthand/or stiffness fiber(s) than “low” strength and/or stiffness fiber(s).

Moreover, the “high” strength and/or stiffness fibers that partiallycomprise each of the second and third layers 120, 130 of the rim 100preferably are of an identical type to the “high” strength and/orstiffness fibers that entirely comprise the fourth layer 140 of the rim.In a currently preferred embodiment of the present invention, thecomposition of a composite-based rim 100 is as shown below in Table I.TABLE I Volume percentage of Volume percentage of “low” strength and/or“high” strength and/or Layer of rim stiffness fibers stiffness fibersFirst, innermost layer About 90% about 10% Second layer About 80% about20% Third layer About 40% about 60% Fourth layer about 0% about 100%Fifth, outermost layer about 0% about 100%

In general, the percentage of “high” strength and/or stiffness fibers inthe first, innermost layer 110 should be lower than the percentage ofsuch fibers in the second layer 120, and the percentage of “low”strength and/or stiffness fibers in the first, innermost layer 110should be higher than the percentage of such fibers in the second layer120. Conversely, the percentage of “high” strength and/or stiffnessfibers in each of the fourth layer 140 and the fifth, outermost layer150 should be higher than the percentage of such fibers in the thirdlayer 130, and the percentage of “low” strength and/or stiffness fibersin the first, innermost layer 110 should be lower than the percentage ofsuch fibers in each of the second layer 120 and the third layer 130.

The percentage of “low” strength and/or stiffness fibers in the firstlayer 110 of the rim generally is in the range of about 81% to 100%, andthe range of “high” strength and/or stiffness fibers in the first layerof the rim is in the range of about 19% to 0%, whereas the percentage of“low” strength and/or stiffness fibers in the fourth layer 140 of therim generally is in the range of about 0% to 39%, and the range of“high” strength and/or stiffness fibers in the fourth layer of the rimis in the range of about 100% to 61%.

The “low” strength and/or stiffness fibers that partially comprise atleast the first, second and third layers 110, 120, 130 of the rim 100generally have a stiffness (i.e., modulus) in the range of about 8 Msito 12 Msi, wherein a stiffness in the range of about 10 Msi to 11 Msi iscurrently preferred, and a stiffness of about 10.5 Msi is currently mostpreferred. These “low” strength and/or stiffness fibers also generallyhave a strength in the range of about 300 Ksi to 500 Ksi, wherein astrength in the range of about 350 Ksi to 400 Ksi is currentlypreferred, and a strength of about about 375 Ksi is currently mostpreferred.

One, some or all of the layers that include “low” strength and/orstiffness fibers may include solely one specific type of “low” strengthand/or stiffness fiber, or may include a plurality of different types ofsuch fibers, but each “low” strength and/or stiffness fiber includedwithin each of these layers generally has strength and/or stiffnesscharacteristics that fall within the above ranges.

Numerous suitable “low” strength and/or stiffness fibers are known;however, current exemplary “low” strength and/or stiffness fibersinclude, but are not limited to, E-glass fiber, which is commerciallyavailable from numerous commercial suppliers (e.g., Owens Corning ofToledo, Ohio, USA), as well as steel wire, which is also commerciallyavailable from numerous suppliers (e.g., Baekaert Corporation ofMarietta, Ga. USA).

The “high” strength/stiffness fibers that partially comprise at leastthe first, second and third layers 110, 120, 130 of the rim 100, andthat entirely comprise at least the rim's fifth layer 150 generally havea stiffness in the range of about 28 Msi to 50 Msi, and a strengthgenerally in the range of about 700 Ksi to 1000 Ksi.

Preferably, the “high” strength and/or stiffness fibers that entirelycomprise at least the fifth layer 150 of the rim 100 have a “higher”strength and/or a “higher” stiffness than the “high” strength and/orstiffness fibers that entirely or partially comprise the fourth layer140, and that partially comprise the first, second and third layers 110,120, 130.

The “higher” strength and/or stiffness fibers generally have a stiffnessin the range of about 38 Msi to 50 Msi and generally have a strength inthe range of about 800 Ksi to 1000 Ksi, wherein stiffnesses in the rangeof about 40 Msi to 44 Msi and strengths in the range of about 800 Ksi to900 Ksi are currently preferred, and wherein stiffnesses in the range ofabout 42 Msi to 43 Msi and a strength of about 800 Ksi are currentlymost preferred.

The “high” strength and/or stiffness fibers generally have a stiffnessin the range of about 28 Msi to 37 Msi and generally have a strength inthe range of about 600 Ksi to 800Ksi, wherein stiffnesses in the rangeof about 32 Msi to 35 Msi and strengths in the range of about 600 Ksi to700Ksi are currently preferred, and wherein stiffnesses in the range ofabout 33 Msi to 34 Msi and a strength of about 700Ksi are currently mostpreferred.

One, some or all of the layers 110, 120, 130, 140, 150 of the rim 100may include solely one specific type of “high” or “higher” strengthand/or stiffness fibers, or may include a plurality of different typesof such fibers. In an exemplary embodiment of the Table I composition,“high” strength and/or stiffness fibers are included among thecomposition of the first, second, third layers 110, 120, 130 of the rim100 and entirely comprise the fourth layer 140 of the rim, while only“higher” strength and/or stiffness fibers comprise the fifth layer 150of the rim.

Numerous suitable “high” and “higher” strength/stiffness fibers areknown. Generally, both the “high” and “higher” strength/stiffness fibersare carbon-based fibers, with currently exemplary “high” strength and/orstiffness fibers including, but not being limited to, T-700 carbonfiber, and currently exemplary “higher” strength/stiffness fibersincluding, but not being limited to, T-800 carbon fiber. Both T-700 andT-800 carbon fibers are commercially available from numerous commercialsuppliers, such as Toray Composites, Inc. of Tacoma, Wash., USA.

The compositions of the layers 110, 120, 130, 140, 150 of the rim 100not only are selected to tailor the strengths and stiffnesses of therim, but also to create a smooth gradient of radial stress and strainfrom layer to layer.

Specifically, by varying the composition (i.e., the volume percentagesof “low,” “high” and “higher” strength/stiffness fibers) of the layers110, 120, 130, 140, 150, not only are the strength and stiffnesses ofthose layers varied, so too are their densities and moduli. The densityand modulus of each layer of the rim 100 are important factors in theequations (see below) that govern flywheel system operation and, moreparticularly, that directly influence generation of radial stress andstrain in the rim.Energy=½*(rim density)*(rim volume)*(rotor radius ofgyration)²*(rotational speed of rim)²Hoop Stress=(rim density)*(rim radius)²*(rim rotationalspeed)²=(strain)*(modulus)Radial Stress˜(rim density)*(rim thickness)²*(rotational speed of rim)²

By having a rim 100 comprised of five layers 110, 120, 130, 140, 150with compositions as set forth above, the rim (during operation) is ableto produce a desirably smooth gradient of radial stresses and strainsfrom the inner layer of the rim to the outer layer of the rim.

This “smooth” gradient is caused by the variation in composition fromlayer to layer of the rim 100, plus the fact that the density of the“low” strength/stiffness fibers is greater than the density of the“high” and “higher” strength/stiffness fibers, while the hoop modulus ofthe “low” strength/stiffness fibers is less than the hoop modulus of the“high” and “higher” strength/stiffness fibers. Therefore, wherein theamount of “low” strength/stiffness fibers decreases from the innermostlayer 110 to the outermost layer 150 of the rim, and the amount of“high” strength and/or stiffness fibers increases from the first layerto the fifth layer, the modulus of each layer also increases from theinnermost layer to the outermost layer. In addition, the density of eachlayer may be reduced from the innermost layer 110 to the outermost layer150. This allows the inner layers of the rim to radially load the rim'souter layers, thus reducing the radial stress and strain in all of therim layers.

Because the composition of each layer does not radically change fromlayer to layer, the density and modulus of each layer do not radicallychange from one layer to the next. This, in turn, creates a “smooth”hoop stress and strain gradient, where the increase from one rim layerto the next is not a sharp increase, but rather a comparatively small,incremental increase. At the same time, the radial stress and strain ofeach layer remains bounded from the inner layer 110 to the outer layer150.

According to the above equations, further control over the radial stressand strain gradient in the rim can be achieved by modifying the radiusof one or more of the layers 110, 120, 130, 140, 150 of the rim 100.

Each layer 110, 120, 130, 140, 150 of the rim 100, however, has apredetermined, non-modified diameter/radius, as shown in FIG. 1B.Generally, the gap 160 has a diameter, D_(G), of about 12.5 inches, and,thus, a radius of about 6.25 inches. The diameter, D₁, of the rim 100 atits first layer 110 is about 13.5 inches; the diameter, D₂, of the rimat its second layer 120 is about 15.5 inches; the diameter, D₃, of therim at its third layer 130 is about 17.6 inches, the diameter, D₄, ofthe rim at its fourth layer 140 is about 19.3 inches; and the diameter,D₅, of the rim at its fifth layer is about 21.1 inches. Therefore, thecurrently preferred, non-modified radii of the rim 100 at its first,second, third, fourth and fifth layers 110, 120, 130, 140, 150 are,respectively, about 6.75 inches, 7.75 inches, 8.8 inches, 9.65 inches,and 10.05 inches.

These radii are currently preferred based on the composition of thelayers 110, 120, 130, 140, 150 of the rim 100. It should be understood,however, that one, some or all of the layers 110, 120, 130, 140, 150 ofthe rim 100 may have radii/diameters that are greater or less than thesecurrently preferred diameters/radii for various reasons (e.g., in orderto allow/facilitate modification or variation of the radial stress andstrain gradients of the rim from layer to layer) without departing fromthe scope of this invention.

Conventional composite-based rims do not achieve this type of smoothgradient due to having thicker layers of fibers, wherein the fiberscomposition is uniform from layer to layer. If, for example, aconventional flywheel system rim is comprised of separate layers thatare each made of the same material, there will be no change in radialstress or strain (i.e., no gradient) from layer to layer. If, instead, aconventional flywheel system rim is comprised of separate layers ofdifferent material, but wherein each layer comprises 100% of aparticular material, then the difference in radial stress and strainfrom layer to layer will be highly pronounced, thus resulting in asharp, pronounced radial stress and strain gradient.

Therefore, in contrast to conventional composite-based rims, a rim 100in accordance with the present invention exhibits a gradual increase instrength and stiffness from its first layer 110 to its fifth layer, andbounds the radial stress and strain from its first to fifth layers. Thisresults in a rim 100 that is much more failure resistant thanconventional composite-based rims, as shown in the following comparativeexamples.

COMPARATIVE EXAMPLES

Two five layer composite-based rims of the present invention were testedfor comparison purposes against three conventional two-layer rims,wherein each layer of the conventional rims was entirely comprised ofT-700 carbon-fibers, and wherein the two layers of the conventional rimswere not commingled.

Also, all five of the rims tested had identical overall dimensions (thusindicating that the radial thickness of each layer of the two-layer rimwas 2.5 times greater than the radial thickness of the five-layer rim ofthe present invention), and were incorporated into otherwise identicalflywheel systems for testing purposes.

One or more cracks were observed to have formed in each of the threeconventional two-layer rims that were tested after less than fifteenhours of continuous operation of the flywheel system at rotationalspeeds of less than 20.5 KPM. Specifically, crack formation was observedin the first conventional two-layer rim tested after 14.17 hours offlywheel system operation at 20.3 KPM, in the second conventionaltwo-layer rim tested after 5 hours of flywheel system operation at 15KPM, and in the third conventional two-layer rim tested after 3.23 hoursof flywheel system operation at 18.4 KPM. Thus, on average, crackformation was observed in these conventional two-layer rims after only7.47 hours of flywheel system operation at 17.9 KPM.

Two five-layer rims 100 having characteristics in accordance with thepresent invention, and having overall dimensions identical to the threeconventional two-layer rims tested were incorporated into otherwiseidentical flywheel systems to the flywheel systems into which theconventional three two-layer rims were incorporated for testing. Crackformation was not observed in either the first five-layer rim after 24hours of flywheel system operation at 23.5 KPM, or the second five-layerrim after 1000 hours of flywheel system operation at 22.5KPM.

According to these results, five-layer rims 100 in accordance with thepresent invention are safer than otherwise identically dimensionedconventional composite-based rims, even if incorporated into otherwiseidentical flywheel systems that were operated at much higher rotationalspeeds for much longer durations as compared to the flywheel systemsinto which the conventional two-layer rims were incorporated. Moreover,this added rotational speed and duration of operation of a flywheelsystem that incorporates a five-layer rim 100 of the present inventionwill translate into the generation of much higher amounts of kineticenergy as compared to a flywheel system that incorporates one of theconventional two-layer rims that was tested.

Additionally, even though these tests indicate that five-layer rimsallow for safe operation of a flywheel system without the formation ofcracks in the rim, the design of a rim in accordance with the presentinvention is such that even if a crack was to form in the rim, the crackwould most likely form in a predetermined layer of the rim, and wouldnot propagate enough to cause failure of the rim prior to the crackbeing detected by safety equipment/monitors.

As noted above, the strength and stiffness to density ratio of eachlayer of the rim increases from the first layer 110 of the rim 100 tothe fifth layer 150 of the rim, while the radial stress and straindecreases from the first layer to the fifth layer. Thus, in the unlikelyevent that a crack does form in a five-layer rim 100 of the presentinvention, it will most likely form in the rim's first, innermost layer,which is the comparatively weakest layer of the rim, and which is thelayer that encounters the highest hoop stresses and strains in relationto its capacity.

And if/when a crack does form in the first layer, the crack will beinhibited from propagating into the second layer because each layer hascommingled fibers, and because the second layer is stronger than thefirst layer and, thus, is more resistant to crack formation/propagation.And if a crack does happen to propagate into the second layer of therim, it will be inhibited from propagating into the stronger thirdlayer, and so on. The fourth and fifth layers, of the rim are especiallyhighly resistant to crack formation/propagation because they arecomprised entirely of one or more “high” strength and/or stiffnessfibers.

Therefore, there will be a significant delay between the onset of acrack in the first, innermost layer of the rim and the propagation ofthat crack through the entire radius of the rim. This delay will be morethan long enough to allow flywheel system safety equipment/monitors todetect that a crack has formed, to trigger a cessation of power to theflywheel system, and for the flywheel system to safely decelerate andspin to a halt prior to rim failure/burst.

The flywheel system is able to detect the onset of a crack because suchan event will cause an upset and/or imbalance condition in the spinningflywheel system, wherein such a condition will be sensed by levelcontrols (not shown) or other equipment incorporated into the flywheelsystem. Upon sensing this condition, the level controls wouldimmediately cause discontinuation of the supply of power to theflywheel, thus causing the flywheel to gradually decelerate to a stop.

Therefore, the five-layer rim compositions of the present inventionrepresent a design philosophy/methodology that not only deters crackformation in the rim by controlling the radial stresses encounteredwithin the rim to manageable levels. Further, such a designphilosophy/methodology, by virtue of the tailored compositions of thelayers of the rim, causes cracks (if any are formed at all) to beinitially formed in the first layer, and severely inhibits the abilityof such cracks to propagate into the second, third, and especiallyfourth and fifth layers of the rim.

This, in turn, allows the flywheel system to be confidently operated athigh speeds up to and above 22.5 KPM without fear of adverse effects(i.e., rim failure/burst), thus allowing for the system to beneficiallygenerate a large amount of kinetic energy.

Referring now to FIG. 2, an apparatus 200 is shown for manufacturing acomposite rim via a filament winding technique in accordance with thepresent invention. The apparatus 200 includes a combined fiberstorage/dispensing device 210 (e.g., a creel), which is equipped with atleast one holder/rack (not shown), for holding a unit (e.g., a spool) offiber material, and for dispensing tows of fiber from one or more of thespools as is generally known in the art.

The number of racks/holders present in the device is greater than or, asis currently preferred, equal to the number of tows that will becombined by the apparatus to form each separate layer of the rim. Tenracks are preferable for practicing the present invention, in which eachlayer of the rim 100 (whether the layer is comprised of solely one typeof fiber, or of a combination of more than one fiber) generally includesten tows, each of which can be supplied from one of the holders. It isunderstood, however, that the number of holders is not crucial, however,as the process can be carried out by one of ordinary skill in the artwithout undue experimentation with greater than or fewer than tenracks/holders.

Fiber is fed from each rack/holder and layered atop each other inpredetermined tow arrangements in order to commingle the tows in eachrim layer. This preliminarily-formed layer 220 emerges from an outputend 230 of the device 210 and is directed through one or more physicaladjustment devices (e.g., one ore more rollers 240 and/or one or morecombs 250) effective to align the fibers within the preliminarily formedlayer, to control the tension of the layer, and/or to provide the layerwith a predetermined bandwidth (i.e., thickness).

The layer 220 then enters a resin treatment area 260 of the apparatus200 where a spinning drum 270 is continuously being impregnated with wetresin 280 from a holding area 290 (e.g., a resin bath). The wet resin280 can be any epoxy suitable for filament winding, or any suitablethermoset or thermoplastic resin system. It is currently preferred toselect the resin such that when it cures it has a minimum tensilestrength of about 4 Ksi.

As the layer 220 advances through the resin treatment area 260, theresin-coated drum 270 spins in the direction of advancement of thelayer, thus causing the bottom-facing side 300 of the fiber layer to becoated with wet resin 280. Optionally, the resin treatment area 260 mayalso include an implement (e.g., a knife 310 ) to control the thicknessof the resin mixture 280 being coated upon the layer 220.

It should be understood that the resin 280 can be introduced to thelayer 220 via different techniques than that which is described aboveand depicted in FIG. 2. By way of non-limiting example, the layer 220can be directed into the resin holding area 290, thus causing it to becoated with resin. Other alternative techniques are generally known inthe art.

The wet layer 220 is then directed through one or more additionalalignment/tensioning devices (e.g., one or more rollers 320 and/or oneor more combs 330) to produce a uniform, wet layer of fiber material ofa predetermined bandwidth. Generally, the bandwidth of each layer is inthe range of about 1.0 inch to 1.3 inch, wherein a currently preferredbandwidth being in the range of about 1.05 inch to about 1.15 inch.

The layer 220 is then fed through a guiding device 340 (e.g., an eyelet)and onto a shape-instilling shell 350 (e.g., a metal-based mandrel),which is rotating in the direction of advancement of the layer. Thelayer 220 is caused to be wound onto the rim shell/base 350 to form oneof the layers of the multiple-layer rim 100 of the flywheel system.

Once all the layers of the rim 100 are in place, the shell 350 is cured(e.g., via an oven), thus causing the resin 280 on each layer to dry,and causing the mandrel 350 to outwardly expand. Thereafter, the mandrel350 is removed from the oven and allowed to cool. During cooling, themandrel 350 (but not the rim) shrinks to its original dimensions, thusfacilitating removal of the rim 100 therefrom. Optionally, objects knownin the art (e.g., peel plies and/or bleeder cloths) may be utilized tofacilitate/expedite the removal of the rim 100 from the shell 350following curing.

In an exemplary embodiment of the present invention, ten tows of fibercomprise each layer of the rim 100, wherein the selection of thespecific tows for inclusion in each layer is made based on the desiredvolume percentage composition of that particular layer. Because thefourth layer 140 of the rim 100 is comprised of 100% “high” strength andstiffness fibers, that layer is generally formed of ten commingled towsof “high” strength and/or stiffness fibers, while because the fifthlayer 150 of the rim is comprised of 100% “higher” strength and/orstiffness fibers, that layer is generally formed of ten commingled towsof “higher” strength and/or stiffness fibers.

The remaining layers 110, 120, 130 of the rim 100 are comprised of “low”strength and/or stiffness and “high” strength and/or stiffness fibers,and are formed, pro rata, from commingled tows of those types of fibers.For example, the first layer 110 of the rim 100 is comprised of about90% “low” strength and/or stiffness fibers and about 10% “high” strengthand/or stiffness fibers and, therefore, is generally formed from ninetows of “low” strength and/or stiffness fibers and one tow of “high”strength and/or stiffness fibers. The second layer 120 of the rim 100 iscomprised of about 80% “low” strength and/or stiffness fibers and about20% “high” strength and/or stiffness fibers and, therefore, is generallyformed from eight tows of “low” strength and/or stiffness fibers and twotows of “high” strength and/or stiffness fibers. The third layer 120 ofthe rim 100 is comprised of about 40% “low” strength and/or stiffnessfibers and about 60% “high” strength. and/or stiffness fibers and,therefore, is generally formed from four tows of “low” strength and/orstiffness fibers and six tows of “high” strength and/or stiffnessfibers.

The layers of the rim that include commingled fiber tows can havevarious tow placement orders. For example, in the first layer 110 of therim 100 it is currently preferred that the one “high” strength and/orstiffness tow be either the first or the tenth tow of the layer, whilein the second layer 120 of the rim, it is currently preferred that atleast one of the two “high” strength and/or stiffness tows be either thefirst or tenth tow, but that both “high” strength and/or stiffness towsnot be placed adjacent each other. In the third layer 130 of the rim100, it is currently preferred that none of the “low” strength tows beplaced adjacent each other.

Further aspects and embodiments of the present invention are depicted inFIGS. 3-7. In FIG. 3, a mandrel 2 is shown for winding resin-impregnatedtows in a fiber band 3 to produce an elongated annular composite “log”4, shown in FIG. 4. The “log” 4 can comprise a one or more annularflywheel rims and can be cut into numerous shorter annular flywheelrims. The tows can be wound in accordance with a winding technique knownin the art, such as a winding technique described and/or depicted inU.S. Pat. No. 4,370,899 to Swartout, or in U.S. Pat. No. 5,628,232 toBosley et al., or in U.S. Pat. No. 5,665,192 to Wolki et al.

The mandrel 2 illustrated has two end flanges 1, which help confine theresin-impregnated fiber tows on the ends of the mandrel 2 as the fiberband 3 is being wound. The length (L_(m)) of the mandrel 2 is the fulllength between the facing surfaces of the two flanges 1.

The fiber band 3 is made up of a number of fiber tows: one example,described below, has twenty fiber tows in the fiber band. The fiber bandis be made up of a mixture of carbon fiber tows and glass fiber towswhich are impregnated with wet resin and wound onto the mandrel by awinding apparatus which traverses back and forth lengthwise of themandrel as the mandrel turns and winds the fiber band in layers onto themandrel. A number of such layers are laid down in a zone, in which theratio of glass fiber tows to carbon fiber tows is constant in eachlayer.

The ratio of glass fiber tows to carbon fiber tows is incrementallyincreased in the next zone or layer of multiple layers to produce a zonewith a greater proportion of carbon fiber tows. The proportion of carbonfiber tows can be further increased in each subsequent zone until thelast zone in which all the tows may be all carbon fiber tows. Forexample, a composite flywheel made in accordance with this approachcould be made in 5 contiguous zones from inside to radially outside, asfollows: 1. 10% CF, 90% GF; 2. 20% CF, 80% GF; 3. 50% CF, 50% GF; 5.100% CF (as used herein, “GF” is glass fiber and “CF” is carbon fiber).

When the fiber band 3 is wound onto the mandrel 2, an undesirabledistribution of glass and carbon fiber tows, can occur, as shown in FIG.5, wherein the carbon fiber tows are radially stacked in aligned regionsor columns 12, separated by regions or columns 14 of radially stackedglass fiber tows, all in an epoxy matrix. The forces action on theflywheel rim during high speed rotation can be substantial and thedifferent modulus of elasticity of the glass and carbon fibers inadjacent regions can result in shear forces between the adjacentregions. These shear forces have never resulted in any known failures ordamage to any flywheel rim, but it is thought best to avoid thepossibility by winding the fiber tows on the mandrel in such a way as todistribute the carbon fiber tows more uniformly amongst the glass fibertows. According to the present invention, the fiber band is wound ontothe mandrel in such a way such that the carbon fiber tows lie in amacroscopically uniform distribution in each zone.

In accordance with the present invention, it has been found that thatthe foregoing can be accomplished by controlling the correlation betweenlead rate of the fiber band as it is wound onto the mandrel per mandrelrevolution and the winding length. Specifically, it has been found thatvarious lay up patterns can be obtained cyclically by changing thewinding length W_(L) while holding constant other parameters such aslead rate L_(R) per revolution of mandrel, mandrel diameter, fiber bandwidth and position of carbon fiber tow(s) within a fiber band of glassfiber tows.

The winding length W_(L) is defined as the traverse distance of fiberband center line between one end of the mandrel 2 and the other endduring winding, as shown in FIG. 4. The lead rate L_(R) is thelongitudinal distance between adjacent turns of a band of fiber,measured center-to-center, as it is wound on the mandrel.

The lead rate L_(R) is often less than the fiber band width since theband are usually made to overlap. To make a good composite rim, thevalue of L_(R) is no greater than the fiber band width. This is the mostpractical way to make a composite rim strong enough in the hoopdirection by laying up fiber axis as close as possible to hoop directionof the rim.

In the case of FIGS. 3-5, the winding parameters are as indicated inTable II below. TABLE II FIG. Winding Length W_(L) (inch) 3 165.5 3166.0 4 165.8 4 165.7 5 165.9 5 165.6

Other parameters are constant regardless of winding parameters. Suchparameters are indicated in Table III below: Parameter Value/MeasurementLead Rate (L_(R)) 1.5 inch/revolution Band Width 3 inches Number oftotal Carbon Fibers in 2 Carbon Fiber tows out of 20 total tows towsNumber of total Glass Fibers in tows 18 Glass Fiber tows out of 20 totaltows Mandrel Diameter 12.45 inches

In an exemplary embodiment according to these parameters, the positionof the two carbon fiber tows in the fiber band correspond to the #1position and the #15 position in a fiber band as shown below, whereinthe remaining positions are occupied by glass fiber tows: 1 2 3 4 5 6 78 9 10 11 12 13 14 15 16 17 18 19 20

Other possible carbon fiber positions according to these parametersinclude, but are not limited to (a) position #6 and position #11, and(b) position #1 and position #16.

The undesirable stacked fiber pattern of FIG. 5 can be avoided, and thedesirable random or uniform carbon fiber tow distribution of FIG. 7 canbe attained by satisfying the following equations:W _(L)=(N+B/A)*L _(R). and W _(L) +L _(R) <L _(M)and M*L _(R) =N*S _(P)wherein:

N=Maximum integer obtained when W_(L) is divided by L_(R)

A=integer larger than B

B=integer smaller than A

B/A≠1, ½, ⅓, ¼

W_(L)=Winding Length (inch)

L_(R)=Lead Rate (inch)

L_(M)=Distance between inner faces of two mandrel flanges (inch)

M=integer≧2

N=integer≧2

S_(P)=fiber space amongst other fibers (inch)

Wet filament winding, where a thermoset resin such as epoxy isimpregnated into raw fibers during the winding operation, is a currentlypreferred fabrication method for a composite rim. The fibers arearranged in tows and the macroscopic distribution of the carbon fibertows is preferably uniform or random throughout the rim. The carbonfibers and glass fibers are concentrated in these tows, so thedistribution of the actual fibers is not uniform or random, but thedistribution of the tows is uniform or random. This is the meaning of“macroscopic” uniform or random distribution.

Although one or more currently preferred embodiments of the inventionhas/have been described using specific terms, such description is forillustrative purposes only, and it is to be understood that changes andvariations may be made without departing from the spirit or scope of thefollowing claims.

1. A hybrid composite flywheel rim comprising: a plurality of layersbeing wound in an annulus on a mandrel; and wherein at least one layerof said plurality of layers being composed of at least two differenttypes of fibers impregnated with a thermosetting resin, said twodifferent fibers having different elastic moduli; one of said two fibertypes being randomly distributed amongst the other fiber macroscopicallyin said at least one layer.
 2. (canceled)
 3. The hybrid compositeflywheel rim of claim 1, wherein at least one of the strength andstiffness of each layer increases in each layer from an innermost layerof the rim to an outermost layer of the rim.
 4. The hybrid composite rimof claim 1, wherein the elastic moduli of the two different elasticmoduli differ by at least 5 Msi.
 5. The hybrid composite flywheel rim ofclaim 1, wherein at least one layer of said plurality of layers beingformed entirely from one of a low modulus fiber and a high modulusfiber, and another layer of said plurality of layers being formed fromboth the low modulus fiber and the high modulus fiber.
 6. A hybridcomposite flywheel rim as defined claim 1, wherein the followingequations are satisfied:W _(L)=(N+B/A)*L _(R). and W _(L) +L _(R) <L _(M) and M*L _(R) =N*S _(P)and wherein: N=Maximum integer obtained when W_(L) is divided by L_(R)A=integer larger than B B=integer smaller than A B/A≠1, ½, ⅓, ¼W_(L)=Winding Length (inch) L_(R)=Lead Rate (inch) L_(M)=Distancebetween inner faces of two mandrel flanges (inch) M=integer≧2N=integer≧9 S_(P)=fiber space amongst other fibers (inch)
 7. The hybridcomposite flywheel rim of claim 1, wherein said one of said two fibertypes is distributed amongst the other fiber in a cross hatch patternmacroscopically.
 8. (canceled)
 9. The hybrid composite flywheel rim ofclaim 7, wherein at least one of the strength and stiffness of eachlayer increases in each layer from an innermost layer of the rim to anoutermost layer of the rim.
 10. The hybrid composite rim of claim 7,wherein the elastic moduli of the two different elastic moduli differ byat least 5 Msi.
 11. The hybrid composite flywheel rim of claim 7,wherein at least one layer of said plurality of layers being formedentirely from one of a low modulus fiber and a high modulus fiber, andanother layer of said plurality of layers being formed from both the lowmodulus fiber and the high modulus fiber.
 12. A hybrid compositeflywheel rim as defined claim 7, wherein the following equations aresatisfied:W _(L)=(N+B/A)*L _(R). and W _(L) +L _(R) <L _(M) and M*L_(R) =N*S _(P)and wherein: N=Maximum integer obtained when W_(L) is divided by L_(R)A=integer larger than B B=integer smaller than A B/A≠1, ½, ⅓, ¼W_(L)=Winding Length (inch) L_(R)=Lead Rate (inch) L_(M)=Distancebetween inner faces of two mandrel flanges (inch) M=integer≧2N=integer≧2 S_(P)=fiber space amongst other fibers (inch)
 13. A hybridcomposite flywheel rim, comprising: a plurality of layers arranged so asto form an annular structure; and wherein at least one layer of saidplurality of layers being composed of at least two fibers havingdifferent elastic moduli, said fibers including carbon fiber, glassfiber, said fibers fixed in a matrix of thermosetting resin, said carbonfiber being distributed amongst the other fiber in a cross hatch patternmacroscopically in said at least one layer.
 14. A hybrid compositeflywheel rim as defined claim 13, wherein the following equations aresatisfied:W _(L)=(N+B/A)*L _(R). and W _(L) +L _(R) <L _(M) and M*L _(R) =N*S _(P)and wherein: N=Maximum integer obtained when W_(L) is divided by L_(R)A=integer larger than B B=integer smaller than A B/A≠1, ½, ⅓, ¼W_(L)=Winding Length (inch) L_(R)=Lead Rate (inch) L_(M)=Distancebetween inner faces of two mandrel flanges (inch) M=integer≧2N=integer≧2 S_(P)=fiber space amongst other fibers (inch)
 15. (canceled)16. The hybrid composite flywheel rim of claim 13, wherein at least oneof the strength and stiffness of each layer increases in each layer froman innermost layer of the rim to an outermost layer of the rim.
 17. Thehybrid composite rim of claim 13, wherein the elastic moduli of saidfibers having different elastic moduli differ by at least 5 Msi.
 18. Acomposite flywheel rim, comprising: an annular structure having aplurality of zones, each with multiple fiber layers in a resin matrix,each said fiber layer having a mixture of carbon fiber tows and glassfiber tows at a ratio of tows that is constant in each layer of anysingle zone, and said ratio incrementally increases zone-by-zoneradially toward outside zones of said rim; and wherein said carbon fibertows lie in a macroscopically uniform distribution in each zone bycontrolling the correlation between lead rate of the fiber band as it iswound onto the mandrel per mandrel revolution and the winding length.19. The hybrid composite flywheel rim of claim 1, wherein thethermosetting resin is an epoxy resin.
 20. A hybrid composite flywheelrim comprising: at least two different types of fibers impregnated witha thermosetting resin and wound in an annulus on a mandrel, said twodifferent fibers having different elastic moduli; one of said two fibertypes being randomly distributed amongst the other fiber macroscopicallyand wherein the thermosetting resin is an epoxy resin.
 21. The hybridcomposite flywheel rim of claim 20, further comprising a plurality oflayers, at least one layer of said plurality of layers being composed ofat least two different types of fibers impregnated with a thermosettingresin, said two different fibers having different elastic moduli; one ofsaid two fiber types being randomly distributed amongst the other fibermacroscopically in said at least one layer.
 22. The hybrid compositeflywheel rim of claim 21, wherein at least one of the strength andstiffness of each layer increases in each layer from an innermost layerof the rim to an outermost layer of the rim.
 23. The hybrid compositerim of claim 21, wherein the elastic moduli of the two different elasticmoduli differ by at least 5 Msi.
 24. The hybrid composite flywheel rimof claim 21, wherein at least one layer of said plurality of layersbeing formed entirely from one of a low modulus fiber and a high modulusfiber, and another layer of said plurality of layers being formed fromboth the low modulus fiber and the high modulus fiber.
 25. A hybridcomposite flywheel rim as defined claim 20, wherein the followingequations are satisfied:W _(L)=(N+B/A)*L _(R). and W _(L) +L _(R) <L _(M) and M*L _(R) =N*S _(P)and wherein: N=Maximum integer obtained when W_(L) is divided by L_(R)A=integer larger than B B=integer smaller than A B/A≠1, ½, ⅓, ¼W_(L)=Winding Length (inch) L_(R)=Lead Rate (inch) L_(M)=Distancebetween inner faces of two mandrel flanges (inch) M=integer≧2N=integer≧2 S_(P)=fiber space amongst other fibers (inch)